change log message to debug

The "sending heartbeat" message is spamming our logs. I'd like to propose to switching that to debug.

Author: Michael Reinsch

lib/kafka/consumer_group.rb

@@ -92,7 +92,7 @@ def commit_offsets(offsets)
     end
 
     def heartbeat
-      @logger.info "Sending heartbeat..."
+      @logger.debug "Sending heartbeat..."
 
       response = coordinator.heartbeat(
         group_id: @group_id,